Editing Synchronous Data Link Control (section)

=== HDLC features not in SDLC ===
Features present in HDLC, but not SDLC, are:
* frames not a multiple of 8 bits long are illegal in SDLC, but optionally legal in HDLC.
* HDLC optionally allows addresses more than 1 byte long.
* HDLC has an option for a 32-bit frame check sequence.
* asynchronous response mode, and the associated SARM and SARME U frames,
* asynchronous balanced mode, and the associated SABM and SABME U frames,
* and several other frame types created for HDLC:
** the selective reject (SREJ) S frame,
** the reset (RSET) command, and
** the nonreserved (NR0 through NR3) U frames.
Also not in SDLC are later HDLC extensions in ISO/IEC 13239 such as:
* 15- and 31-bit sequence numbers,
* the set mode (SM) U frame,
* 8-bit frame check sequence,
* a frame format field preceding the address,
* an information field in mode set U frames, and
* the "unnumbered information with header check" (UIH) U frame.
<!--This list is not exhaustive.-->